Referring to FIG. 1, a schematic, cross-sectional view of a portion of a conventional photovoltaic module 1 is shown. The module 1 includes a number of photovoltaic cells 2 formed on substrate 3. Generally, each cell 2 has a first transparent electrode layer 4, a photoactive layer 5, including a p-type absorber-generator sublayer and an n-type, collector-converter sublayer, and a second electrode layer 6 which can be formed from metal.
Sunlight passes through the substrate 3 and the first, transparent electrode layer 4 and is converted to electricity in the photoactive layer 5. Electrical current thus generated flows between adjacent cells 2 via electrode portions 7 which extend across interconnect region 8. As shown, the cells 2 are connected in electrical series, i.e., the "top" electrode of one cell 2 is connected to the "bottom" electrode of an adjacent cell 2, to enhance the power output of the module 1. Stated another way, the anode of one cell 2 is connected to the cathode of an adjacent cell 2.
The module 1 can be economically manufactured by forming corresponding layers of the cells 2 continuously and then cutting through certain layers as desired to divide the module 1 into individual cells 2 and provide for series connections between cells 2. The process of dividing the module layers into separate cells is referred to as "isolation" herein. Construction of the module 1 can include the following steps wherein the sequence of certain steps can be varied: forming a substantially continuous sheet of transparent electrode material on substrate 3; forming a substantially continuous sheet of photoactive material, which can include a plurality of sub-layers forming a heterojunction, on the transparent electrode sheet; cutting through the photoactive material and the transparent electrode material to expose the substrate 3 (in region A); cutting through the photoactive material to expose a portion of the transparent electrode material (in region B); forming a second substantially continuous electrode sheet over the transparent electrode material and photoactive material such that the second electrode sheet forms an electrical contact with the exposed portion of the transparent electrode material; and cutting through the second electrode sheet (in region C) to provide final separation of individual cells 2.
One problem associated with conventional methods for isolating and interconnecting cells such as described above is that it is difficult to selectively cut through certain layers without severing or damaging adjacent layers as may be desired during construction. In this regard, it will be appreciated that the individual layers of photovoltaic cells are ordinarily very thin, sometimes only several microns thick or less. Accordingly, the layer cutting techniques employed must be highly accurate. However, in order to provide a module which can compete with alternate energy sources, it is desirable to minimize construction costs. Therefore, manufacturers of photovoltaic modules have sought inexpensive yet accurate techniques for selectively cutting through module layers for isolation and interconnection and/or methods which are less sensitive to accuracy requirements.
One technique for selectively removing layers involves applying sacrificial, strippable lines of material at selected positions beneath layers which are to be removed during construction. Thus, for example, in FIG. 1, a sacrificial, strippable line of material can be applied on layer 4 in region B. This line of material can later be stripped by sandblasting or other methods thereby removing overlying layers such as the photoactive layer 5 of FIG. 1. Even though these methods may be accurate, reproducible, and controllable, the complete removal of all strippable material is still difficult and there is a limit to the optical loss minimization based on both the strippable material itself and the removal method.
In addition, known techniques for selectively removing layers in connection with cell isolation and interconnection generally result in production of relatively wide interconnecting regions between adjacent cells. For example, in one known module, the interconnection regions are at least about 0.040 inches wide due to a practical limitation in the layer removal step. The power production efficiency of a module can be enhanced by increasing the ratio of the productive surface area of the module to the total surface area of the module exposed to sunlight. Accordingly, it is desirable to reduce the width of the interconnection regions thereby increasing the proportion of the exposed module surface covered by productive photovoltaic cells.
Further, some known techniques for use in constructing modules are relatively sensitive to small errors in positioning cut lines used for cell isolation and interconnection. For example, when using such techniques, if certain cut lines are positioned too far to the left or right of the desired position, an open circuit may be formed or an insulating material may be interposed between elements intended for interconnecting cells in electrical series such that the usefulness of the entire module is greatly reduced or substantially eliminated.